[lld-macho][nfc] Set test min version to 11.0

The arm64-apple-macos triple is only valid for versions >= 11.0. (If
one passes arm64-apple-macos10.15 to llvm-mc, the output's min version is still
11.0). In order to write tests easily for both target archs, let's up the
default min version in our tests.

Reviewed By: #lld-macho, thakis

Differential Revision: https://reviews.llvm.org/D124562